At present, a charge status indicator is always provided on an AC-DC charger for displaying charge status to a user. Traditional AC-DC charger is implemented with secondary-side feedback control Flyback switching mode power supply (SMPS). Charge status indicator used in such traditional AC-DC charger is implemented by a red and green two-color common-cathode LED 223, a PNP transistor 222, a silicon rectifier diode 220 and resistors 219, 221, 224 (referring to FIG. 1).
When the product of the output current Io of VOUT and the resistance of resistors 219, V 219 (that is the voltage drop on the resistors 219), is higher than the VBE (about 0.6 V) of the PNP transistor 222, that is, Io*R 219>0.6V, the PNP transistor 222 is turned on in a saturated mode, and the red LED is lighten. Due to the saturated voltage drop VCE of PNP transistor 222 is about 0.3 V, and the turn on voltage drop of the red LED in LED 223 is about 1.6 V, and the sum of both is about 1.9 V, which is lower than the sum 2.3 V of the voltage drop 0.6V on the resistors 219 and the turn on voltage drop 1.7 V of the green LED in LED 223, that is VCE+VR<V219+VG, so the green LED isn't lighten.
When the product of the output current Io of VOUT and the resistance of resistors 219, V 219 (that is the voltage drop on the resistors 219), is lower than the VBE (about 0.6 V) of the PNP transistor 222, that is, Io*R 219<0.6V, the PNP transistor 222 is turned off, and the green LED is lighten.
However, the said circuit according the prior art is not a good choice for AC-DC charger implemented with primary-side feedback control Flyback SMPS because of the following reasons:
1. When a rated load is outputted, the voltage on capacitor 213 is higher than VOUT for 0.8V-1.0V. If primary-side feedback control Flyback SMPS is employed, which will result in that the no-load voltage of output end will higher than rated load voltage for 0.8-1V. Accordingly such circuit can't be adopted in primary-side feedback control Flyback SMPS, and only can be adopted in secondary-side feedback control Flyback SMPS.
2. As such circuit is used to detect the voltage drop generated when charging current is passing through the resistors 219 and diode 220, which result in low efficiency.
3. Too many electronic components are employed in such circuit, which result in higher costs and lower reliability.
Accordingly, a simple and reliable charge status indicator circuit which can be applied in primary-side feedback control Flyback SMPS.